Within the oil and gas industry, the continuing search for and exploitation of oil and gas reservoirs has resulted in the development of directionally drilled exploration and production well boreholes, that is boreholes which extend away from vertical and which permit the borehole to extend into the reservoir to a greater extent than with conventional vertical well boreholes.
Directionally drilled boreholes are now being drilled deeper, longer and higher in angle (from vertical) than previously, with boreholes now being drilled horizontally for considerable distances through the reservoir. Indeed, in some cases the horizontal step out from the surface location of the drilling site may be in excess of 10 kilometers.
It will be recognised that in vertical or near vertical wellbores, most if not all of the tubulars, e.g. drilling tubulars, or string will normally be hanging in tension and apply little in the way of side forces on the wellbore. By contrast, in some high angle or horizontal wellbores, the majority of the lower portion of the tubulars or string will instead be lying on the low side of the borehole with their weight acting on the borehole wall, and generating considerable torsional friction when the tubulars are rotated from surface.
As the horizontal section of the borehole is extended, this torsional friction component increases the applied torque required from surface to rotate the tubulars, to the point where the tubulars are no longer able to transmit sufficient torque to rotate the lower portion of the assembly and to provide power to the drilling process.
A number of methods of reducing rotational torsional losses in the horizontal section of a borehole have been developed. In some instances, friction reducers have been used in the drilling fluids. Alternatively, or additionally, friction reducing non-rotating collars or stabiliser sleeves may be installed on the tubulars used in the horizontal section. In some instances, friction reducing collars or non-rotating stabiliser sleeves may be installed as part of a sub-based tool installed between the drill pipe connections. In other instances, friction reducing collars or non-rotating stabiliser sleeves may be attached to the tubular body of the drill pipe by means of a re-joinable split joint or by means of a clamp.
Each of the above proposed methods for reducing friction and/or drag nevertheless suffer from drawbacks. For example, the provision of a separate sub-based tool provided between tubular joints results in a spacing of 30 feet (9.2 meters) between tools. The requirement for a separate sub also means that the length of tubular handled at the rig floor and in the stacking area is increased, thereby increasing handling time, failure potential and maintenance costs. The requirement for a separate sub also increase the number of connections in a given length or string of drilling tubulars, again increasing handling time, failure potential and maintenance costs.
In the case of split sleeves or clamped on devices, their complexity adds to the risk of failure, to the handling time for installation and/or removal. In some instances, it has been known that such tools can become detached and lost in the hole, requiring workover operations at significant expense to the operator.
In order to gain any substantial benefit from torsional friction reducing devices such as those described above in long horizontal sections of borehole, it is necessary to run considerable numbers of these devices to ensure that the majority of the drilling tubulars in the horizontal section of borehole are supported off the low side of the borehole and rotate in robust efficient bearings. This results in numerous points of contact between the drilling tubulars and the low side of the borehole, each of which increase friction and requiring additional torque from surface.